1. Field of Invention
The present invention relates to a differential gear assembly that may obtain a large speed change ratio using four gears.
2. Background of Related Art
Today, an actuator for changing a rotary motion obtained from a rotary drive source such as a motor into a linear motion has been used in various kinds of mechanical devices. In general, the actuator decelerates a rotation of a rotary shaft through a speed change gear assembly and changes it into the linear motion under a condition that a greater drive torque or a higher rotation is obtained. Accordingly, the speed change gear assembly is one of the important constituent elements of the actuator. Conventionally, in general, involute gears have been assembled for the speed change gear assembly. However, a backlash is inevitable in the assembly such as cycloid gears, involute gears and the like. In case of the actuator which requires a high positional precision, an influence of the backlash is large to degrade the positional precision.
Therefore, the present inventor has invented a speed change assembly that may obtain a large speed change ratio without an inevitable backlash and disclosed its detail in the Japanese Patent Publication No. Hei 7-56324. Since gears used in the interior of the speed change assembly employa so-called Coriolis motion, the speed change assembly will be referred to as a Coriolis gear assembly. FIG. 15 shows a sectional side view of the primary part of the Coriolis gear assembly by the present inventor. In the Coriolis gear assembly, an input gear shaft 1 and an output gear shaft 2 are connected to each other through first to fourth gears A, to A.sub.4. The speed reduction is realized by these gears. The first to fourth gears A.sub.1 to A.sub.4 are bevel gears. Then, the second gear A.sub.2 and the third gear A.sub.3 are provided on a rotary member 3. The rotary member 3 is pivoted on a slant portion la of the input shaft 1. If the rotary member 3 is thus supported obliquely, it is possible to generate a Coriolis motion, to be described later, of the rotary member 3 in accordance with a rotation of the input shaft 1.
Also, rollers 4 and inscribed surfaces 5 are used as gear teeth of each gear so that the rotation of the rollers 4 may absorb a sliding motion generated in the cogged engagement. Accordingly, even if the setting of the backlash is omitted and a pre-pressure is intentionally applied to the gears, it is possible to avoid heat generation caused by the engagement of the gears. In addition, a cross-sectional shape of the rotary member 3 is formed into a U-shape so that portions forming the U-shape may be resiliently deformed to impart the pre-pressure by their resiliency. Then, when the rotary motion of the input shaft 1 is transmitted to the output shaft 2, a two-stage speed reduction effect is attained by the first and second gears A.sub.1 and A.sub.2 and the third and fourth gears A.sub.3 and A.sub.4.
By the way, when the rotary motion of the input shaft is transmitted to the output shaft, the conventional planetary gear assembly (for example, S-C-P type, where S is the sun gear, C is the carrier and P is the planetary gear) is subjected to an one-stage speed reduction effect only. The speed reduction ratio that may be designed is limited. In view of this fact, it is understood that the Coriolis gear assembly presented by the present inventor may be designed to have a greater speed reduction ratio in a wider range and may be applied more broadly. It is an object of the present invention is to aim at further broadening an application range of the Coriolis gear assembly by making possible the forms of the Coriolis gear diversified while utilizing the above-described advantages of the Coriolis gear.